This invention relates to the field of radio transport of alphanumeric messages to and from remote units and at least one base station, and specifically to a method and means for coordinating inbound transmissions to the base station wherein collisions between coded messages and acknowledgments on an inbound channel to the base station are minimized. The present application is related to the instant assignee's U.S. Pat. No. 4,646,082 issued Feb. 24, 1987, entitled "Inbound Acknowledgment Stack" and invented by James R. Engel, et al.
Portable Data terminal systems have been developed for providing data message communications over a conventional radio frequency (RF) link in order to couple remote units, or terminals, to at least one base station, and ultimately to a host computer, to provide operational command and control for a public service agency or the like. Such RF communication systems typically operate on two channels, namely, an inbound channel and an outbound channel to the base station. Thus, although the remote units typically operate in half-duplex mode by transmitting on the inbound channel to the base station and receiving on the outbound channel, the base station transmits on its outbound channel and receives on its inbound channel by utilizing a full-duplex mode.
The operation of this type of system is sequenced and controlled from the fixed end by a "system controller". This system controller organizes and generates the various outbound transmissions which generally include at least a header and a coded message as a single packet of information over the operating channel of the system. Each remote unit that receives an outbound transmission from the base station is generally required to send an acknowledgment (ACK). This acknowledgement may be a positive or affirmative type, which indicates the message has been received and will be acted upon, or a negative acknowledgement indicating at least the address or header was received by not the message. Or, alternatively, it may indicate both header and message were received but that the addressee rejects or is unable to respond to the particular message at this time. In either case, it will means that the same is not or cannot be properly decoded at this time. The base station will repeat its message to the various remote units until an ACK of whatever type has been received.
One known improvement utilizes an imbedded inhibit signal in the outbound transmission to prevent other remote units from beginning a possibly interfering transmission when one is in progress. A second known improvement is the Inbound Acknowledgment Stack which is effective in preventing the simultaneous transmission of acknowledgment signals after a period of a relatively long inhibit. In implementing such a method and arrangement, several difficulties are introduced which increase the likelihood that collisions will occur in such a contention RF communications system.
The first problem is that when two or more remote units are transmitting simultaneously at least one, and often both, inbound transmissions are lost.
A second type of difficulty with the prior art system is the inordinately long delay encountered before transmitting an inbound message when the status of the outbound channel is unknown.
A third type of difficulty occurs while waiting for frame sync, namely, bunching of messages which causes an increase in collisions, both of the message-message type and of the message-ACK type.